The present invention concerns an optical device with an absorption gradient intended to assure homogeneous lighting in the image plane of a lens assembly. The invention also concerns an optical device of this type forming a spectral filter intended to filter selectively spectral bands in the total optical spectrum passing through said optical device. Finally the invention concerns a lens assembly and a camera fitted with an optical device of the aforementioned type.
In order to make a good quality lens assembly for a digital camera, a spectral filter has to be incorporated to remove the red and infrared to which the semiconductor type optical sensors which are fitted to such cameras are very sensitive. An absorption gradient filter also has to be incorporated to homogenise the lighting in the image plane of the lens assembly. Indeed, the camera lens assemblies which are currently available on the market often have a problem of lighting uniformity in their image plane, which results in a substantial reduction in the lighting from the centre of the image plane to the edges thereof. This problem is linked to the fact that the lenses of which such lens assemblies are formed are often spherical lenses. Such lenses are commonly used because they are simple to manufacture by grinding and polishing using abrasive powders. Unfortunately, these lenses are the most inadequate from the point of view of lighting as will be explained with reference to FIG. 1 annexed to the present Application.
The aforementioned FIG. 1 shows two points respectively designated by the alphabetic references A and B. These two points A and B are in the image plane 1 of a spherical lens 2. As FIG. 1 shows, point A is situated on optical axis 4 of lens 2, whereas point B is situated at a distance from axis 4. The projection factor or useful aperture of lens 2 for points A and B will be called respectively xcex1 and xcex2. It is to be noted that the useful aperture becomes smaller for a point which is situated outside the axis, in other words xcex2 less than xcex1. Consequently, the lighting, i.e. the energy per surface unit, decreases for a point situated outside the axis. This is confirmed by the fact that lighting E varies in accordance with the ratio E=Eocos4xcfx89, where E is the incident lighting and xcfx89 is the angle between the optical axis and the straight line linking a point of the image plane at the centre of the lens. Thus, for a point situated on the optical axis, xcfx89=0 and E=Eo is maximum. Conversely, for a point situated outside the optical axis, angle xcfx89 increases and the resulting lighting E decreases.
In order to attempt to compensate for the unfavourable effects of spherical lenses from the point of view of lighting, several solutions can be envisaged. One of these solutions consists in using aspherical lenses or combinations of a large number of spherical lenses. Another known solution consists in using an absorption gradient filter. Such a filter generally takes the form of a substantially plane substrate arranged in or in proximity to the actual image plane or an intermediate image plane. According to a first alternative embodiment, one of the faces of the substrate is coated with a thin film of reflective material such as aluminium. This film has the particularity of being thicker and thus more reflective, at its centre than in its peripheral zone. It thus enables the lighting in the image plane of the lens assembly to be homogenised by reducing the lighting at the centre of the image plane, whereas it allows more light to pass on the edges of said image plane. According to another embodiment, the absorption gradient filter is made of a spun-dyed material whose optical density decreases from the centre towards the edges of the filter. Other even more complex absorption gradient filters can also be used.
The corrective measures which have just been described have the drawback of being expensive. In particular, the lens assembly has to be mounted in a clean environment such as a clean room, and the filters have to be encapsulated in order to be protected from dust, which means high costs. Moreover, integration of two additional elements in the optical path of the lens assemblies means that the lens assemblies cannot be miniaturised and compact. Finally, far from the optical axis of the lens assembly, the angle formed by the incident light with the surface of the absorption gradient filter increases, which causes an increase in the quantity of light reflected by the surface of said filter, and thus a decrease in the light which reaches the optical sensor fitted to digital cameras.
The object of the present invention is to overcome the above problems and drawbacks by providing an optical device which allows the functions of spectral filter and absorption gradient filter to be combined in the same element.
The present invention therefore concerns an optical device intended to homogenise the lighting in an image plane, in particular defined by a digital optical sensor, characterised in that it is formed of a block of transparent filtering homogenous material, this block having a substantially uniform absorption coefficient for all the wavelengths of the visible optical spectrum passing through it, and having a variable thickness suited to a determined absorption profile.
As a result of these features, the present invention provides an optical device whose thickness, suitably selected, generates gradual absorption, allowing uniform luminosity in the image plane of the lens assembly fitted with such a device to be obtained. Compared to absorption gradient filters of the prior art, as a result of its monolithic structure, the device according to the invention is simple and inexpensive to manufacture. It is also compact, so that it can easily be integrated into a lens assembly.
According to a complementary feature of the invention, the block of transparent filtering material forms a lens.
According to another feature of the invention, the optical device has a high value absorption coefficient for wavelengths comprised in a determined spectral band.
Thus, unlike the prior art wherein, in order to make a good quality lens assembly for a digital camera, an absorption gradient filter, a spectral filter and a lens had to be mounted in the optical path of said lens assembly, the present invention enables the functions of these two filters and the lens to be joined in the same optical element. This allows the cost price of the lens assembly to be limited, on the one hand by reducing the number of parts necessary for the mounting thereof, and on the other hand by simplifying manufacture, since only the space separating the optical device according to the invention from the image sensor has to be encapsulated in order to guarantee efficient protection against dust.
The present invention also concerns a photographic lens assembly including an optical device of the aforementioned type, as well as a camera fitted with such a lens assembly.